Injection Molding of Crosslinking Polymers Using Strain Data

ABSTRACT

Non-time dependent calculated variables based on measured strain are used to effectively determine an optimal hold profile for an expanding crosslinking polymer part in a mold cavity. A system and/or approach may first inject molten expanding crosslinking polymer into a mold cavity, then measure strain at the mold cavity or at another location within the injection molding system, and then calculate at least one non-time dependent variable during an injection molding cycle. Next, the system and/or method commences a hold profile for the part, and upon completing the hold profile, the part is ejected from the mold cavity, whereupon a cure profile is commenced.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional, and claims the benefit of thefiling date of U.S. Provisional Patent Application No. 62/523,067, filedJun. 21, 2017, entitled “Injection Molding of Crosslinking PolymersUsing Strain Data.” The entire contents of U.S. Provisional ApplicationNo. 62/523,067 is hereby incorporated by reference.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to injection molding and, moreparticularly, to injection molding of expanding crosslinking polymersand using measured strain to control the injection molding of expandingcrosslinking polymers.

BACKGROUND

Injection molding is a technology commonly used for high-volumemanufacturing of parts constructed from thermoplastic materials. Duringrepetitive injection molding processes, a thermoplastic resin, typicallyin the form of small pellets or beads, is introduced into an injectionmolding machine which melts the pellets under heat and pressure. Themolten material is then forcefully injected into a mold cavity having aparticular desired cavity shape. The injected plastic is held underpressure in the mold cavity and subsequently is cooled and removed as asolidified part having a shape closely resembling the cavity shape ofthe mold. A single mold may have any number of individual cavities whichcan be connected to a flow channel by a gate that directs the flow ofthe molten resin into the cavity.

Expanding crosslinking polymers (e.g., ethylene-vinyl acetate or “EVA”)are one class of polymers that are commonly injection molded. A typicalinjection molding process of expanding crosslinking polymers generallyincludes four basic operations. First, the plastic is heated in theinjection molding machine to allow the plastic to flow under pressure.When injection molding expanding crosslinking polymers, at this step,the polymer is heated to a temperature that is below an activationtemperature of the polymer, or the temperature at which expansion andcrosslinking within the polymer begins to occur.

Next, the melted plastic is injected into a mold cavity or cavitiesdefined between two mold halves that have been closed. The mold orcavity temperature is set to a value that is high enough to activate achemical reaction or reactions that cause the polymer to begin expansionand crosslinking. At a third step, the plastic is held under pressure toallow adequate crosslinking and expansion (or blowing) to occur in thecavity or cavities. Last, the mold halves are opened, and the moldedarticle is removed or ejected from the mold, thereby allowing theplastic to expand to a final shape and configuration that is larger thanthe internal volume of the mold cavity.

In conventional systems, a fixed, predetermined volume of plastic isinjected into the mold cavity. This volume only partially fills thecavity. The mold cavity is then heated to cause a chemical reaction,upon which the plastic is then left to expand to fill the mold cavityand crosslink for a specified hold time.

After the part is ejected, it is quickly removed from the mold to astabilization tunnel where curing occurs. By quickly removing the partfrom the mold, the part can fully expand, and will not be deformed dueto the material being constrained from expanding at areas where the partis still captured in the mold. During the curing phase, the part isallowed to slowly cool to a temperature near room temperature. Excessinternal gases will slowly escape from the part.

The time when the plastic is ejected (which is dependent on thecalculated hold time) is determined or calculated to provide theinjected plastic sufficient time to expand and crosslink (thus beingsufficiently hardened) to the desired final shape so the plastic doesnot deform or become otherwise damaged. However, due to material andmachine variances, using a fixed hold time as the determining variablecan result in varying internal peak cavity pressures, which can impactcrosslinking and expansion while in the mold cavity. Specifically, thechemical reaction that causes the part to expand is less consistent, asevidenced by both delayed and inconsistent pressure-builds in existingsystems. In turn, when the part is ejected from the mold and enters acuring stage where the molded parts continue to expand and crosslinkuntil reaching a final form, expansion and crosslinking may occur atvarying rates, thus resulting in inconsistently sized parts. Further,the parts may have unsightly blemishes and other undesirable flaws.

For example, a melted plastic may have slightly different materialcharacteristics in subsequent injection cycles, thus if subsequentinjection cycles were to depend on prior hold times, the occurrence ofpart imperfections, faults, and other irregularities may arise. If apart is held in the cavity longer than needed, the overall injectionmolding cycle is unnecessarily long, thus the injection molding machineconsumes excess energy which in turn increases operating costs andadversely impacting production capacity. Further, the molded parts maynot experience consistent heat transfer in the mold, which can result ina non-uniform skin layer. The cell structure of the molded part may alsobe non-uniform, meaning free radical molecules may not be aligned. Whenthese molecules are uniformly distributed, the resulting part has moreconsistent an stable dimensions and physical properties.

Further, conventional systems typically do not provide uniform heatdistribution throughout the plastic during the molding process due tovarying mold thicknesses. By unevenly heating the plastic, differentregions of the plastic within the mold cavity can expand at differentrates, which can result in inconsistent parts having wide tolerances.

Further, the molded parts may be incorrectly dimensioned (meaning, partsmay be either too large or too small) and may potentially be too soft ortoo resilient due to insufficient crosslinking. As a result, the moldedpart may fail any number of objective tests such as an abrasion test, acompression set test, and/or a dynamic elasticity test where energy lossis measured over a number of closely timed impacts with a controlledload.

Using non-time dependent measured variables to control an injectionmolding system for expanding crosslinking polymers addresses theproblems identified above. However, some direct sensors to measurenon-time dependent variables, such as sensors placed within a moldcavity, leave undesirable marks on part surfaces. For example, demandfor injection molded parts with high gloss finishes has been increasing,and direct sensors positioned in the mold cavity have a tendency to marthe high gloss finish of the parts, requiring post-molding operations tomachine or otherwise mask or remove the marred regions from the parts.As a result, indirect sensors that are not located in the mold cavityare preferable. Additionally, when the molding system is being used tomake products for medical applications, contact between a sensor and thethermoplastic material may be prohibited.

SUMMARY

Embodiments within the scope of the present invention are directed tothe use of non-time dependent measured variables, as calculated frommeasured strain, to effectively determine an optimal hold profile of oneor more expanding crosslinking polymer parts being formed in a moldcavity. A system and/or approach may first inject molten expandingcrosslinking polymer into a mold cavity, then measure strain at a moldcavity or another location within the injection molding system, and thencalculate from the measured strain at least one non-time dependentvariable during an injection molding cycle. Next, the system and/ormethod commences a hold profile for the part, and upon completing thehold profile, the part is ejected from the mold cavity, whereby thesystem and/or method commences a cure profile for the part.

In these examples, the mold cavity is nearly completely filled at aninjection stage. A suitable hold profile commences when at least onecalculated non-time dependent variable reaches a first threshold value,and continues until the calculated at least one non-time dependentvariable(s) reaches a second threshold value. During this period,additional molten expanding crosslinking polymer is restricted frombeing injected into the mold cavity.

In some examples, the first threshold value is indicative of the moldcavity being substantially full of molten expanding crosslinkingpolymer. The second threshold value may be indicative of the part beingstructurally sound, and being ready to be ejected.

In some examples, the calculated variable is a cavity pressure value. Inthese examples, the first threshold value may be a nominal increase incavity pressure. The second threshold value may be indicative of asubstantially constant cavity pressure value over a specified period oftime. Other examples of threshold values with respect to cavity pressuremeasurements are possible.

In other examples, the calculated variable is a temperature value. Inthese examples, the first threshold value may be a nominal increaseabove an initial cavity temperature. The second threshold value mayrepresent a substantially constant cavity temperature value over aspecified period of time. Other examples of threshold values withrespect to cavity temperature measurements are possible.

In some examples, commencement of the cure profile includes first,calculating from measured strain a different non-time dependentvariable. Upon the calculated different non-time dependent variablereaching a third threshold value, the cure profile is ended. In theseexamples, the third threshold value may be indicative of the part beingstructurally sound. The calculated different non-time dependent variablecomprises a pressure value.

In other examples, commencing the cure profile includes allowing thepart to cool for a predetermined amount of time.

In some approaches, an expanding crosslinking polymer injection moldingsystem includes an injection molding machine comprising an injectionunit and a mold forming at least one mold cavity, a controller adaptedto control operation of the injection molding machine, and one or moresensors coupled to the injection molding machine and the controller. Theinjection unit is adapted to receive and inject a molten expandingcrosslinking plastic material into the at least one mold cavity to forma molded part. At least one of the one or more sensors is adapted tomeasure strain during the injection mold cycle. The controller isadapted to commence a hold profile for the expanding crosslinkingpolymer part, and is further adapted to cause the molded part to beejected from the mold cavity upon completing the hold profile, whereupona cure profile then commences.

By optimizing the hold profile, consistent parts having minimal defectsand variances in size are produced. Calculations of the non-timedependent variable or variables based on strain data can be used as ahighly accurate measure of when to make process parameter decisions.Further, due to the consistency in molded parts produced when using theoptimized hold profile, the subsequent cure profile may further ensurethat molded parts remain consistent and within tight tolerances (e.g.,within tolerances of approximately 2 mm).

BRIEF DESCRIPTION OF THE DRAWINGS

The above needs are at least partially met through provision of one,more than one, or any combination of the approaches for injectionmolding expanding crosslinking polymers described in the followingdetailed description, particularly when studied in conjunction with thedrawings, wherein:

FIG. 1 illustrates an elevation view of an exemplary injection moldingmachine having two strain gauges and a controller coupled thereto inaccordance with various embodiments of the present disclosure;

FIG. 2 illustrates an example relationship between a blowing agent and acrosslinking agent over time during injection molding of an expandingcrosslinking polymer in accordance with various embodiments of thepresent disclosure; and

FIG. 3 illustrates an example relationship between screw position,cavity pressure, and melt pressure during an expanding crosslinkingpolymer injection molding cycle in accordance with various embodimentsof the present disclosure.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions and/or relative positioningof some of the elements in the figures may be exaggerated relative toother elements to help to improve understanding of various embodimentsof the present invention. Also, common but well-understood elements thatare useful or necessary in a commercially feasible embodiment are oftennot depicted in order to facilitate a less obstructed view of thesevarious embodiments. It will further be appreciated that certain actionsand/or steps may be described or depicted in a particular order ofoccurrence while those skilled in the art will understand that suchspecificity with respect to sequence is not actually required. It willalso be understood that the terms and expressions used herein have theordinary technical meaning as is accorded to such terms and expressionsby persons skilled in the technical field as set forth above exceptwhere different specific meanings have otherwise been set forth herein.

DETAILED DESCRIPTION

An injection molding process for expanding crosslinking polymers isherein described. An example of an expanding crosslinking polymer isEVA, which, when polymerized, includes any number of blowing agents andany number of crosslinking agents which are activated by temperatures.For example, the blowing agents and crosslinking agents may be activatedbetween approximately 160° C. and approximately 190° C., or preferably,between approximately 165° C. and approximately 185° C., and morepreferably, between approximately 170° C. and approximately 180° C.,which may provide an optimal range for blowing and crosslinking tooccur. Other examples are possible.

As illustrated in FIG. 1, an injection molding machine 100 that moldsexpanding crosslinking polymers includes an injection unit 102 and aclamping system 104. The approaches described herein may be suitable forvertical press injection molding machines and any other known types ofinjection molding machines. The injection unit 102 includes a hopper 106adapted to accept the expanding crosslinking polymer material in theform of pellets 108 or any other suitable form. In many of theseexamples, the pellets 108 include any number of foaming agents,crosslinking agents, and the like. Other examples are possible.

The hopper 106 feeds the pellets 108 into a heated barrel 110 of theinjection unit 102. Upon being fed into the heated barrel 110, thepellets 108 may be driven to the end of the heated barrel 110 by areciprocating screw 112. The heating of the heated barrel 110 and thecompression of the pellets 108 by the reciprocating screw 112 causes thepellets 108 to melt, thereby forming a molten plastic material 114. Themolten plastic material 114 is typically processed at a temperatureselected within a range between about 110° C. and about 150° C. Thismelt temperature is below an activation of the molten plastic material114.

The reciprocating screw 112 advances forward and forces the moltenplastic material 114 toward a nozzle 116 to form a shot of plasticmaterial 114 which will ultimately be injected into a mold cavity 122 ofa mold 118 via one or more gates 120 which direct the flow of the moltenplastic material 114 to the mold cavity 122. In other embodiments, thenozzle 116 may be separated from one or more gates 120 by a feed system(not illustrated). The mold cavity 122 is formed between the first andsecond mold sides 125, 127 of the mold 118 and the first and second moldsides 125, 127 are held together under pressure via a press or clampingunit 124. The mold 118 may include any number of mold cavities 122 toincrease overall production rates. The shapes and/or designs of thecavities may be identical, similar, and/or different from each other.

The press or clamping unit 124 applies a predetermined clamping forceduring the molding process which is greater than the force exerted bythe injection pressure acting to separate the two mold halves 125, 127,thereby holding together the first and second mold sides 125, 127 whilethe molten plastic material 114 is injected into the mold cavity 122. Tosupport these clamping forces, the clamping system 104 may include amold frame and a mold base, in addition to any other number ofcomponents.

The reciprocating screw 112 continues forward movement, causing the shotof molten plastic material 114 to be injected into the mold cavity 122.The mold cavity 122 is heated to a temperature that is higher than theactivation temperature of the molten plastic material 114. For example,the mold cavity 122 may be heated to between approximately 160° C. andapproximately 185° C., and preferably, between approximately 170° C. and180° C. As such, a chemical reaction begins to occur within the moltenplastic material 114 as it contacts sidewalls of the mold cavity 122. Itis understood that walls of the mold cavity 122 may be preheated priorto injection the molten plastic material 114, or alternatively, may berapidly heated to a suitable temperature as the molten plastic material114 enters the mold cavity 122. Examples of heating techniques that maybe used to heat surfaces of the mold that define the mold cavity are:Resistive heating (or joule heating), conduction, convection, use ofheated fluids (e.g., superheated steam or oil in a manifold or jacket,also heat exchangers), radiative heating (such as through the use ofinfrared radiation from filaments or other emitters), RF heating (ordielectric heating), electromagnetic inductive heating (also referred toherein as induction heating), use of thermoelectric effect (also calledthe Peltier-Seebeck effect), vibratory heating, acoustic heating, anduse of heat pumps, heat pipes, cartridge heaters, or electricalresistance wires, whether or not their use is considered within thescope of any of the above-listed types of heating.

The injection molding machine 100 also includes a controller 140 whichis communicatively coupled with the machine 100 via connection 145, andis generally used to control operation of the injection molding machine100. The connection 145 may be any type of wired and/or wirelesscommunications protocol adapted to transmit and/or receive electronicsignals. In these examples, the controller 140 is in signalcommunication with at least one sensor, such as, for example, a strainsensor 128 located in the nozzle 116 and/or a strain sensor 129 locatedon an external surface of mold 118. The strain sensor 129 may be secureddirectly to an external surface of the mold 118 or may be secured to anexternal surface of the mold 118 by an assembly. An example of a strainsensor located in a nozzle of an injection molding system is disclosedin U.S. patent application Ser. No. 15/615,996, filed Jun. 7, 2017 andentitled “Upstream Nozzle Sensor for Injection Molding Apparatus andMethods of Use”, which is hereby incorporated by reference. An exampleof a strain sensor secured directly to an external surface of a mold isdisclosed in U.S. patent application Ser. No. 15/216,754, filed Jul. 22,2016 and entitled “Method of Injection Molding using One or More StrainGauges as a Virtual Sensor,” which is hereby incorporated by reference.An example of a strain sensor secured on an external surface of a moldby an assembly is disclosed in U.S. patent application Ser. No.15/448,992, filed Mar. 3, 2017 and entitled “External Sensor Kit forInjection Molding Apparatus and Methods of Use”, which is herebyincorporated by reference.

Returning to FIG. 1, it is understood that any number of additionalsensors capable of sensing any number of characteristics of the mold 118and/or the machine 100 may be placed at desired locations of the machine100. In particular, a temperature sensor may be placed on the machine100, and the data obtained from the temperature sensor may be used inconjunction with measured strain from strain sensor 128 or strain sensor129. Further, although two strain sensors 128 and 129 are depicted inFIG. 1, a single strain sensor or more than two strain sensors may beprovided on the machine 100.

The controller 140 can be disposed in a number of positions with respectto the injection molding machine 100. As examples, the controller 140can be integral with the machine 100, contained in an enclosure that ismounted on the machine, contained in a separate enclosure that ispositioned adjacent or proximate to the machine, or can be positionedremote from the machine. In some embodiments, the controller 140 canpartially or fully control functions of the machine via wired and/orwired signal communications as known and/or commonly used in the art.

The strain sensor 128 generates a signal which is transmitted to aninput of the controller 140. The controller 140 is in communication witha virtual cavity sensor 141, which is implemented as a program, or a setof software instructions. In this disclosure, the term “virtual cavitysensor” refers to a module that can calculate the value of a non-timedependent variable, such as pressure within mold cavity 122, withoutdirectly measuring this non-time dependent variable. If the strainsensor 128 is not located within the nozzle 116, the controller 140 canbe set, configured, and/or programmed with logic, commands, and/orexecutable program instructions to provide appropriate correctionfactors to estimate or calculate values for the measured characteristicin the nozzle 116.

Likewise, the strain sensor 129 generates a signal which is transmittedto an input of the controller 140 and the virtual cavity sensor 141. Ifthe strain sensor 129 is not provided on the mold cavity 122, thecontroller 140 can be set, configured, and/or programmed with logic,commands, and/or executable program instructions to provide appropriatecorrection factors to estimate or calculate values for the measuredcharacteristic at the end-of-fill position.

It is understood that any number of additional sensors may be used tosense and/or measure operating parameters including but not limited tostrain. For example, U.S. patent application Ser. No. 15/198,556, filedon Jun. 30, 2016 and published as U.S. Publication No. 2017/0001356,describes a sensor positioned prior to the end-of-fill to predict theend-of-fill and is hereby incorporated by reference in its entirety.

The controller 140 is also in signal communication with the screwcontrol 126. In some embodiments, the controller 140 generates a signalwhich is transmitted from an output of the controller 140 to the screwcontrol 126. The controller 140 can control any number ofcharacteristics of the machine, such as, for example, injectionpressures (by controlling the screw control 126 to advance the screw 112at a rate which maintains a desired melt pressure of the molten plasticmaterial 114 in the nozzle 116), barrel temperatures, clamp closingand/or opening speeds, cooling time, inject forward time, hold profiles,overall cycle time, pressure set points, ejection time, cure profiles,screw recovery speed, and screw velocity. Other examples are possible.

The signal or signals from the controller 140 may generally be used tocontrol operation of the molding process such that variations inmaterial viscosity, mold cavity 122 temperatures, melt temperatures, andother variations influencing filling rate are taken into account by thecontroller 140. Adjustments may be made by the controller 140 in realtime or in near-real time (that is, with a minimal delay between sensors128, 129 sensing values and changes being made to the process), orcorrections can be made in subsequent cycles. Furthermore, severalsignals derived from any number of individual cycles may be used as abasis for making adjustments to the molding process. The controller 140may be connected to the sensors 128, 129, the screw control 126, and orany other components in the machine 100 via any type of signalcommunication known in the art or hereafter developed.

The controller 140 includes virtual cavity sensor 141 adapted to controlits operation, any number of hardware elements 142 (such as, forexample, a memory module and/or processors), any number of inputs 143,any number of outputs 144, and any number of connections 145. Thevirtual cavity sensor 141 may be loaded directly onto a memory module ofthe controller 140 in the form of a non-transitory computer readablemedium, or may alternatively be located remotely from the controller 140and be in communication with the controller 140 via any number ofcontrolling approaches. The virtual cavity sensor 141 includes logic,commands, and/or executable program instructions which may contain logicand/or commands for controlling the injection molding machine 100according to a mold cycle. The virtual cavity sensor 141 may or may notinclude an operating system, an operating environment, an applicationenvironment, and/or a user interface.

The hardware 142 uses the inputs 143 to receive signals, data, andinformation from the injection molding machine being controlled by thecontroller 140. The hardware 142 uses the outputs 144 to send signals,data, and/or other information to the injection molding machine. Theconnection 145 represents a pathway through which signals, data, andinformation can be transmitted between the controller 140 and itsinjection molding machine 100. In various embodiments this pathway maybe a physical connection or a non-physical communication link that worksanalogous to a physical connection, direct or indirect, configured inany way described herein or known in the art. In various embodiments,the controller 140 can be configured in any additional or alternate wayknown in the art.

The connection 145 represents a pathway through which signals, data, andinformation can be transmitted between the controller 140 and theinjection molding machine 100. In various embodiments, these pathwaysmay be physical connections or non-physical communication links thatwork analogously to either direct or indirect physical connectionsconfigured in any way described herein or known in the art. In variousembodiments, the controller 140 can be configured in any additional oralternate way known in the art.

As previously stated, during an injection molding cycle, the strainsensors 128, 129 are adapted to measure strain related to operation ofthe machine 100. The virtual cavity sensor 141 is configured tocalculate from the measured strain at least one non-time dependentvariable during the injection mold cycle. Although the arrangementdepicted shows two strain sensors 128 and 129 for measuring strain, thevirtual cavity sensor 141 can calculate at least one non-time dependentvariable based on data from a single strain sensor or from two or morestrain sensors. The virtual cavity sensor 141 may calculate the at leastone non-time dependent variable using non-strain data as well, such astemperature data provided by a temperature sensor. During operation, thecontroller 140 commences a hold profile which may be stored in thevirtual cavity sensor 141. In some examples, the hold profile may becommenced upon the calculated variable reaching a threshold value. Uponcompleting the hold profile, the controller 140 will send a signal tothe machine that causes the mold cavity 122 to open and to eject thepart from the mold 118 so that it can commence the cure profile, wherenecessary continued expansion and crosslinking occurs to form astructurally sound molded part. For example, a structurally sound moldedpart may be free of divots, dwells, flash, partial fills, burns, tears,minimal imperfections such as sink marks and/or swirls on the surfacelayer, weakness at thickness changes, and should have uniformity ofmechanical properties.

In these examples, the variable or characteristic may be one other thantime (e.g., a cycle, step, or any other time), thus time is not directlymeasured and used to determine the length of the hold profile, andaccordingly, when to eject the part. Rather, the variable orcharacteristic relies on another value or indicator as a determiningfactor for part readiness. The use of one or more non-time dependentvariables is advantageous because during successive runs, even with thesame supply of pellets 108, variations in pellet quality, catalyststability, ambient conditions, or other factors may influence thecross-linking of the polymer material from shot-to-shot. While atime-dependent process may provide satisfactory parts most of the time,a system that determines ejection readiness based on one or morenon-time dependent variables is preferable, as this provides a moreaccurate assessment for each individual shot or run of the moldingsystem.

Turning to FIG. 2, which illustrates an example relationship between theblowing agent and the crosslinking agent of the expanding crosslinkingpolymer over time, during the injection molding process, the blowingagent first activates at a given temperature and begins to react overtime. Generally speaking, the blowing agent, depicted by the solid linein FIG. 2, will cause the part to expand, thus dictating the part size.At approximately the same point that the blowing agent is activated, thecrosslinking agent, depicted by the dashed line in FIG. 2, activates andbegins to form structural bonds within the polymer. Both the blowingagent and crosslinking agent generate exothermic reactions, thus theygenerate heat as the reaction advances, which in turn causes the blowingand crosslinking agents to continue their respective chemical reactions.When the blowing process concludes, the reaction will stop emittingheat. At this point, crosslinking continues until the part issufficiently formed, meaning the molten plastic material 114 is nolonger in a flowable state.

Referring again to FIG. 1, upon the molten plastic material 114substantially filling the mold cavity 122, a hold profile is commenced.During the hold profile, which may commence upon the calculated variable(which can be calculated by the virtual cavity sensor 141 from strainmeasured by any of strain sensors 128 and/or 129) reaching a firstthreshold value, additional molten plastic material 114 is restrictedfrom being injected into the mold cavity 122. This may occur by shuttingoff the supply of molten plastic material 114, or alternatively, bycontrolling movement of the screw 112. Additionally, the mold cavity 122is held closed during the hold profile. Upon the calculated variable(which can be calculated by the virtual cavity sensor 141 from strainmeasured by any of sensors 128 and/or 129) reaching a second thresholdvalue, controller 140 causes the hold profile to end, whereby the moldcavity 122 is opened and the part is ejected from the mold 118 and thecure profile to commence.

Turning now to FIG. 3, which represents an example expandingcrosslinking polymer injection molding cycle 300, the measured variablemay reach first and second threshold values. Line 302 depicts theposition of the screw 112 under a certain injection pressure (i.e.,5,000 psi) once the cavity pressure is built to a desired and/ordesignated trigger point. As an example, the pressure can decrease fromapproximately 5,000 psi to approximately 2,000 psi at this point. Inthis example, during injection of the expanding crosslinking polymer,melt pressure, which is depicted by line 304, is first increased andthen held to a substantially constant value. Accordingly, the calculatedvariable (which can be calculated by the virtual cavity sensor 141 fromstrain measured by any of strain sensors 128 and/or 129) may be a meltpressure. As a non-limiting example, the melt pressure may be betweenapproximately 0 psi and approximately 11,000 psi. Other examples ofsuitable melt pressures are possible. Further, it is understood that insome examples, the melt pressure may not be held to a substantiallyconstant value.

In FIG. 3, line 306 depicts the calculated variable as a cavity pressurevalue. In the illustrated example, in region I, the calculated cavitypressure value (which can be calculated by the virtual cavity sensor 141from strain measured by any of strain sensors 128 and/or 129) exceedsthe first threshold value. As previously noted, in some examples, duringthe injection molding process, the mold cavity 122 can be essentiallycompletely filled with molten plastic material 114.

In this example, the calculated cavity pressure value is defined as acavity pressure greater than a nominal value, which may be at leastpartially caused by the molten plastic material 114 completely fillingthe mold cavity 122 and exerting a pressure on the cavity walls. Theincrease in cavity pressure may additionally or alternatively be causedby the expansion of the molten plastic material 114 within the moldcavity 122. It is understood that in some examples, the first thresholdvalue may be any desired quantity. For example, the first thresholdvalue may be a distinct cavity pressure value, such as, approximately100 psi. Other examples are possible.

Upon the virtual cavity sensor 141 calculating a calculated cavitypressure value exceeding the first threshold value, the controller 140commences the hold profile. As illustrated by line 304 in FIG. 3, themelt pressure is then adjusted (for example, reduced). In theillustrated example, the melt pressure is again held to a substantiallyconstant value, such as, for example, between approximately 500 psi andapproximately 3,500 psi. Other examples are possible. This pressure ismaintained by controlling movement of the screw 112 to a hold pressuremeasured at the nozzle by the sensor 128.

At region II, as the melt pressure is maintained, the calculated cavitypressure increases as the molten plastic material 114 begins to blow orexpand. Upon the virtual cavity sensor 141 calculating a cavity pressurevalue that exceeds the second threshold value, the hold profile iscompleted, and the controller 140 causes the part to be ejected from themold cavity 122. As an example, the second threshold value may be adistinct cavity pressure value, such as, between approximately 100 psiand approximately 2,000 psi. Other examples are possible. This secondthreshold value is indicative of the expanding crosslinking polymericpart being sufficiently structurally sound to complete its expansion andcrosslinking outside of the mold cavity. At this point, the mold cavity122 is opened, thus the melt pressure drops to approximately 0.

In some examples, the calculated variable calculated by the virtualcavity sensor 141 from measured strain is a cavity temperature value.Accordingly, in these examples, the first threshold value may be acavity temperature value that is representative of the mold cavity 122being substantially completely filled. For example, the first thresholdtemperature value may be between approximately 168° C. and approximately176° C. Other examples are possible. Similarly, in these examples, thesecond threshold value may be a cavity temperature value that isrepresentative of the molten plastic material 114 being sufficientlystructurally sound for ejection. In these examples, the cavitytemperature may plateau or decrease at a point when the part is ready tobe ejected from the mold cavity 122. As a non-limiting example, thesecond threshold temperature value may be between approximately 160° C.and approximately 180° C. Other examples are possible.

Because the mold cavity 122 is substantially completely filled (e.g.,between approximately 95% and approximately 99% fill) prior tocommencement of the hold profile, and because pressure is applied to themolten plastic material 114 thereby holding it against the heated wallsof the mold cavity 122, heat is uniformly distributed or transferred tothe molten plastic material 114 due to the increased surface contact.Advantageously, the blowing and crosslinking agents will activate moreuniformly, thus forming more cohesive bonds.

So configured, the hold profile can be described as the combination ofregions I and II in FIG. 3. The injection molding machine 100 does notcontemplate the actual duration of time required to commence the holdprofile, and rather, the machine 100 operates in a closed loop moldholding pattern. So configured, molded parts have more consistent partsizes and appearances, as well as a uniform skin layer due to consistentheat transfer. Further, not only will particular parts have consistentdimensions, the hold profile helps to ensure reliability and consistencyacross a range of sizes of parts, which has been particularlychallenging with respect to expanding crosslinking polymer articles.Further still, the hold profile provides better control over theprocess, allowing the part to dictate when the cavity is full and readyto be ejected. In some examples, using the hold profile can decrease theoverall cycle time due to a reduced cure time. Additionally, the use ofthe hold profile can generate parts having more uniformity in cellstructure due to free radical molecules becoming aligned. As such, thehold profile makes a more consistent and stable dimensioned part, withconsistent physical properties.

At region III, the controller 140 commences a cure profile. Asillustrated in FIG. 3, the cavity pressure will ultimately plateau asthe part ceases to further expand. Upon the virtual cavity sensor 141calculating a calculated cavity pressure value that exceeds a thirdthreshold value, the cure profile is completed, and the part is ejected,removed from the cavity 122 or the entire machine 100, and transferredto a stabilization tunnel where curing occurs. As an example, the thirdthreshold value may be a different cavity pressure value, such as,between approximately 2,000 psi and approximately 4,000 psi. Otherexamples are possible. Alternatively, the third threshold value may be apredetermined rate of change in pressure values, which may indicate thatthe pressure is no longer increasing. Other examples are possible. Thisthird threshold value is indicative of the expanding crosslinkingpolymeric part being essentially fully formed and ready for furtherprocessing. By using the third threshold value to determine the durationof the cure profile, the machine 100 will not prematurely eject partsthat have not fully cured. Additionally, the machine 100 reducesinefficiencies by using unnecessarily long cure times, which can consumeunnecessary power and reduce overall yields of the machine.

In some examples, the cure profile may be commenced for a fixed,predetermined time necessary for the part to be fully cured. Forexample, the cure profile may be programmed to last betweenapproximately 100 seconds and approximately 450 seconds. Other examplesare possible. The machine 100 is capable of using a fixed period of timefor the cure profile due to the use of the optimized hold profile, whichforms consistent parts having uniform characteristics, such as internalcrosslinking and bond strength. This uniformity at the onset of the cureprofile will result in continued uniformity during the cure profile.

Those skilled in the art will recognize that a wide variety ofmodifications, alterations, and combinations can be made with respect tothe above described embodiments without departing from the scope of theinvention, and that such modifications, alterations, and combinationsare to be viewed as being within the ambit of the inventive concept.

The patent claims at the end of this patent application are not intendedto be construed under 35 U.S.C. § 112(f) unless traditionalmeans-plus-function language is expressly recited, such as “means for”or “step for” language being explicitly recited in the claim(s). Thesystems and methods described herein are directed to an improvement tocomputer functionality, and improve the functioning of conventionalcomputers.

What is claimed is:
 1. A method for determining whether an expandingcrosslinking polymer part is ready to be ejected from a mold cavityduring an injection molding cycle of an injection molding system, themethod comprising: injecting molten expanding crosslinking polymer intoa mold cavity; measuring strain at the mold cavity or at anotherlocation within the injection molding system during the injectionmolding cycle; calculating from the measured strain at least onenon-time dependent variable during the injection mold cycle; commencinga hold profile for the expanding crosslinking polymer part; uponcompleting the hold profile, ejecting the molded part from the moldcavity; and commencing a cure profile for the molded part.
 2. The methodof claim 1, wherein commencing the hold profile comprises: upon thecalculated at least one non-time dependent variables reaching a firstthreshold value, commencing the hold profile; restricting additionalmolten expanding crosslinking polymer from being injected into the moldcavity; and upon the calculated at least one non-time dependentvariables reaching a second threshold value, terminating the holdprofile.
 3. The method of claim 2, wherein the first threshold value isindicative of the mold cavity being substantially full of moltenexpanding cross linking polymer.
 4. The method of claim 2, wherein thesecond threshold value is indicative of the part being structurallysound.
 5. The method of claim 2, wherein the calculated at least onenon-time dependent variable comprises a cavity pressure value.
 6. Themethod of claim 1, wherein the hold profile commences at a substantiallyconstant pressure.
 7. The method of claim 1, wherein commencing the cureprofile comprises: further calculating from the measured strain adifferent non-time dependent variable; and upon the calculated differentnon-time dependent variables reaching a third threshold value,terminating the cure profile.
 8. The method of claim 7, wherein thethird threshold value is indicative of the part being structurallysound.
 9. The method of claim 7, wherein the calculated differentnon-time dependent variable comprises a pressure value.
 10. The methodof claim 1, wherein commencing the cure profile comprises allowing thepart to cool for a predetermined amount of time.
 11. An expandingcrosslinking polymer injection molding system comprising: an injectionmolding machine comprising an injection unit and a mold forming a moldcavity, the injection unit adapted to receive and inject a moltenexpanding crosslinking plastic material into the mold cavity to form amolded part; a controller adapted to control operation of the injectionmolding machine; and one or more sensors coupled to the injectionmolding machine and the controller; wherein at least one of the one ormore sensors is adapted to measure strain during the injection moldcycle, wherein the controller is adapted to commence a hold profile forthe expanding crosslinking polymer part and is further adapted to causethe molded part to be ejected from the mold cavity upon completing thehold profile and commence a cure profile for the molded part.
 12. Thesystem of claim 11, wherein the controller commences the hold profile bycommencing the hold profile when at least one non-time dependentvariable calculated from measured strain reaches a first thresholdvalue, restricting additional molten expanding crosslinking polymer frombeing injected into the mold cavity, and terminating the hold profilewhen the calculated at least one non-time dependent variables reaches asecond threshold value.
 13. The system of claim 12, wherein the firstthreshold value is indicative of the mold cavity being substantiallyfull of molten expanding cross linking polymer.
 14. The system of claim12, wherein the second threshold value is indicative of the part beingstructurally sound.
 15. The system of claim 12, wherein the calculatedat least one non-time dependent variable comprises a cavity pressurevalue.
 16. The system of claim 11, wherein the hold profile commences ata substantially constant pressure.
 17. The system of claim 11, whereincommencing the cure profile comprises: calculating a different non-timedependent variable from measured strain; and upon the measured differentnon-time dependent variables reaching a third threshold value,terminating the hold profile.
 18. The system of claim 17, wherein thethird threshold value is indicative of the part being structurallysound.
 19. The system of claim 17, wherein the calculated differentnon-time dependent variable comprises a pressure value.
 20. The systemof claim 11, wherein commencing the cure profile comprises allowing thepart to cool for a predetermined amount of time.